A "local terminal" is defined herein to mean a communication device which is the initiator of an attempt to establish a communication channel with another communication device. A "remote terminal" is defined herein to mean a communication device which is the responder to an attempt by the local digital terminal to establish a communication channel.
For example, a secure telephone unit (STU) is capable of establishing a "secure" communication channel with another STU. Secure communications comprises encrypting, transmitting, receiving and decrypting data. The modem training procedure begins when one STU "initiates" the establishment of the secure communication channel (e.g., the user of one STU presses the "secure" button).
Communication system 101 comprises local terminal 110, modems 114, 138, radio units 118, 132, 134, public switched telephone network 141 (PSTN), analog links 112, 140, 150, RF digital links 120', digital links 116, 136 and remote terminal 152. PSTN 141 comprises, for example, communication satellites 131 and terrestrial telephone networks 142, 146, 148 (TTNs). TTNs 142, 146, 148 may communicate with communication satellites 131 via RF analog links 144. TTNs 142, 146, 148 may alternatively be inter-connected via wirelines (not illustrated in FIG. 1). Links 112, 116, 136, 140 and 150 are wireline links and links 144 are RF satellite links. Radio unit 118 is coupled to radio unit 134 via RF digital network 120'.
Local digital terminal 110 and remote analog terminal 152 produce digital bitstreams modulated by internal modems (not illustrated, FIG. 1), providing modulated carriers transmissible via analog links 112 and 150, respectively. For example, when local digital terminal 110 and remote analog terminal 152 are STUs, their internal modems may produce encrypted modulated carriers. Modulated carriers received via analog links 112 and 150 are demodulated by the internal modems of local digital terminal 110 and remote analog terminal 152 to produce digital bitstreams that may be processed by terminals 110, 152.
Modems 114, 138 modulate incoming digital bitstreams and demodulate incoming modulated carriers. As a result, signals communicated via analog links 112, 140, 150 are modulated carriers and signals communicated via RF digital networks 120' and digital links 116, 136 are digital bitstreams.
Signals communicated via RF digital networks 120' may be limited to bandwidths as low as 2400 bits per second (bps). For example, local digital terminal 110, analog link 112, modem 114, digital link 116 and radio unit 118 may be located off shore, or may be mobile land- or air-based units or may be fixed. Radio unit 134, digital link 136, modem 138, analog links 140, 150, PSTN 142 and remote analog terminal 152 may be land-based equipment. Additionally, signals communicated via RF digital network 120' are processed to include error correction/detection information and this is done by processing data from several frames of digitized information. The net effect is for digital network 120' to add group delay of on the order of a half-second to the transmitted digital signals.
A "system delay" is defined herein to mean a delay associated with a communication path between system nodes (e.g., radio unit 118, PSTN 142 etc.) or between a system node and other communication apparatus. Multiple delays and system nodes may exist within local digital terminal 110 and/or between local digital terminal 110 and interworking function (IWF) 138. Additional delays may exist between IWF 138 and remote analog terminal 152.
To establish a digital communication link within conventional communication system 101, the internal modem of local digital terminal 110 must "train" with the internal modem of remote analog terminal 152 to adaptively equalize the line and set near and far echo taps for echo cancellation. This is all performed digitally within the internal modems as part of the modem training procedure. Near the beginning of the modem training procedure, messages describing modem capabilities may be exchanged between internal modems so that the internal modems may determine a desired data rate, among other things. For some modes of operation, capabilities messages need not be exchanged.
FIG. 2 represents the timing of modem training messages exchanged between local digital terminal 110 and remote analog terminal 152 for an operative configuration of conventional communication system 101. As used in FIG. 2 (and FIGS. 5 and 6 and associated text, infra), "tx" is an abbreviation for "transmit" and "rx" is an abbreviation for "receive".
Referring also to FIG. 1 and associated text, the modem training procedure is initiated by local digital terminal 110. Modems 114 and 138 increase the propagation time of the modem training signals through communication system 101. For example, modem 114 requires approximately 0.5 seconds to detect local modem tone 210 (referred to also as LMT). Other than adding delay, modems 114 and 138 are transparent during the modem training procedure between local digital terminal 110 and remote analog terminal 152.
Local digital terminal 110 transmits local modem tone 210 to remote analog terminal 152 beginning at time 240. For example, local modem tone 210 may be a 2100 Hz tone of limited duration. Local digital terminal 110 continues transmission of local modem tone 210 until local digital terminal 110 begins reception of remote modem tone 215 (referred to also as RMT).
Remote analog terminal 152 receives local modem tone 210 beginning at time 245. Remote analog terminal 152 may then wait a certain signaling delay time and transmit remote modem tone 215 to local digital terminal 110 beginning at time 250. For example, remote modem tone 215 may be a P1800 Hz tone of limited duration. A P1800 (or "Pseudo" 1800) Hz tone consists of alternations of dibits 00 and 10, corresponding to +45 degree and -45 degree phase shifts, respectively.
Local digital terminal 110 receives remote modem tone 215 beginning at time 255. First response time-out interval 260, monitored by local digital terminal 110, begins at time 240 when local digital terminal 110 starts transmitting local modem tone 210. Local digital terminal 110 "fails the call" (e.g., hangs up) if it does not begin receiving remote modem tone 215 within first response time-out interval 260. Alternatively, local digital terminal 110 may re-initiate the modem training procedure to attempt to establish communications with remote analog terminal 152.
Capabilities messages 220, 225 are exchanged by local digital terminal 110 and remote analog terminal 152 indicating the "capabilities" of each terminal's internal modem. The exchanged capabilities messages 220, 225 are interpreted according to a predetermined hierarchy to arrive at negotiated parameters (e.g., data rate, etc.) which determine how further communications will be handled. Capabilities messages 220, 225 contain information the terminals use to select a common mode of operation (e.g., a negotiated data rate of 4800 bits per second).
Local digital terminal 110 transmits local capabilities message 220 (referred to also as LCM) beginning at time 275. Remote analog terminal 152 receives local capabilities message 220 beginning at time 280.
Remote analog terminal 152 transmits remote capabilities message 225 (referred to also as RCM) beginning at time 285. Remote capabilities message 225 is received by local digital terminal 110 beginning at time 290. Second response time-out interval 270, monitored by local digital terminal 110, begins at time 275, when local digital terminal 110 starts transmitting local capabilities message 220. Local digital terminal 110 fails the call if it does not begin receiving remote capabilities message 225 within second response time-out interval 270. Alternatively, local digital terminal 110 may re-initiate the modem training procedure to attempt to establish communications with remote analog terminal 152.
FIG. 3 is a flow diagram of a prior art protocol for local terminal modem training and capabilities message exchange. Referring also to FIGS. 1 and 2 and associated text, local terminal modem training and capabilities message exchange begins (block 310) when local digital terminal 110 transmits local modem tone 210 (block 315). Local digital terminal 110 starts an internal timer (block 320) when it begins transmission of local modem tone 210. Local digital terminal 110 then determines whether the internal timer value exceeds first response time-out interval 260 (block 325). When the internal timer value exceeds first response time-out interval 260 (block 325), local digital terminal 110 assumes remote analog terminal 152 is nonexistent or incapable of establishing communications and local digital terminal 110 fails the call (block 355), thus terminating the modem training procedure. For example, 3.3+/-0.7 seconds is a standard first response time-out interval within the telecommunications industry.
When the internal timer value does not exceed first response time-out interval 260 (block 325), local digital terminal 110 determines whether remote modem tone 215 has been received (block 330). When remote modem tone 215 has not been received (block 330), local digital terminal 110 again determines whether first response time-out interval 260 has been exceeded (block 325). The procedure then iterates as shown in FIG. 3.
When remote modem tone 215 has been received (block 330), local digital terminal 110 transmits local capabilities message 220 (block 335). Local digital terminal 110 starts an internal timer (block 340) when it begins transmission of local capabilities message 220. Local digital terminal 110 then determines whether the internal timer value exceeds second response time-out interval 270 (block 345). For example, 2.2 seconds is a standard second response time-out interval within the telecommunications industry.
When the internal timer value exceeds second response time-out interval 270 (block 345), local digital terminal 110 assumes that remote analog terminal 152 is inoperative and local digital terminal 110 fails the call (block 355), thus terminating the modem training procedure.
When the internal timer value does not exceed second response time-out interval 270 (block 345), local digital terminal 110 determines whether remote capabilities message 225 has been received (block 350). When remote capabilities message 225 has not been received (block 350), local digital terminal 110 again determines whether second response time-out interval 270 has been exceeded (block 345). The procedure then iterates as shown in FIG. 3.
When remote capabilities message 225 has been received (block 350), local digital terminal 110 continues the modem training procedure (block 360) at the negotiated data rate and in accordance with the requirements of the internal modems of local digital terminal 110 and remote analog terminal 152.
FIG. 4 is a flow diagram of a prior art protocol for remote terminal modem training and capabilities message exchange. Referring also to FIGS. 1 and 2 and associated text, remote terminal modem training and capabilities message exchange begins (block 410) when remote analog terminal 152 receives local modem tone 210 (block 415). Remote analog terminal 152 then waits a required signaling delay time (block 420). For example, a required signaling delay time may be zero seconds (no delay) or one second. After the required signaling delay time has expired (block 420), remote analog terminal 152 transmits remote modem tone 215 (block 425).
Remote analog terminal 152 then determines whether local capabilities message 220 has been received (block 440). When local capabilities message 220 has not been received, remote analog terminal 152 continues to monitor incoming data until local capabilities message 220 is received. When remote analog terminal 152 receives local capabilities message 220 (block 440), remote analog terminal 152 transmits remote capabilities message 225 (block 445). Remote analog terminal 152 then continues the modem training procedure (block 450) at the negotiated data rate and in accordance with the requirements of the internal modems of local digital terminal 110 and remote analog terminal 152.
A signal transmitted by a local digital terminal (e.g., local digital terminal 110, FIG. 1) in response to a signal received from an analog or other terminal (e.g., remote analog terminal 152) experiences a time delay before it is received by a remote analog terminal (e.g., remote analog terminal 152, FIG. 1). The time delay results from the signal propagation time to, and also signal processing time within, the local digital terminal and transmission delays back to the remote analog terminal. The propagation delay length depends on the distances between the terminals and delays within intervening equipment.
In some settings, interoperability performance requirements dictate that digital and analog STUs support a secure communications link between a digital STU (e.g., local digital communications terminal 110, FIG. 1) and an analog STU (e.g., remote analog communications terminal 152) via PSTN 142. Additionally, it is not uncommon encounter system delays between a terminal (e.g., local digital terminal 110, FIG. 1) and a PSTN interface (e.g., modem 138, FIG. 1). Therefore, a message may be transmitted through multiple devices and links before reaching a final destination. Each device and link contributes additional time delay to the total message propagation time.
Table 1 summarizes approximate timing delays inherent in the inoperative configuration of conventional communication system 101 (FIG. 1). As used in Table 1, "start" is the element number in FIG. 1 where the delay originates and "end" is the element number in FIG. 1 where the delay ends.
TABLE 1 ______________________________________ CONVENTIONAL COMMUNICATION SYSTEM TIMING DELAYS start end delay explanation delay ______________________________________ 110 114 local tone detect time .5 sec 114 138 dig. wireless net. del. .6 sec 138 152 two satellite hops .6 sec 152 138 two satellite hops .6 sec 138 114 dig. wireless net. del. .6 sec 114 detect of remote tone .1 sec 114 110 modem processing delay .1 sec total 3.1 sec ______________________________________
The total round trip delay of up to 3.1 seconds for a system having a digital local terminal 110 exceeds the standard second response time-out interval of 2.2 seconds. Thus, the prior art signaling/training protocol does not work with the digital wireless network delay plus two satellite hops between a digital local terminal 110 and an analog remote analog terminal 152.
With only two total satellite hops between local digital terminal 110 and remote analog terminal 152, the total round trip delay is 3.1 seconds. Therefore, the prior art protocol does not work with two analog terminals 110, 152.
A significant drawback of the prior art protocol is that modem training response time-out intervals for a particular communications system may limit the total delay allowable within local digital terminal 110 and also between local digital terminal 110 and remote analog terminal 152 to as little as one or two satellite hops and then may not accommodate additional timing delays encountered in digital RF networks. However, for some applications, communications ability across a combination of satellite hops and digital RF network delays is desirable.
Thus, what are needed are (i) a practical, economical method and (ii) a similar apparatus, each allowing successful modem training to occur when a digital local terminal 110 is attempting to reliably establish communication with an analog remote terminal, or vice versa. What are particularly needed are (i) a modem training method and (ii) a similar apparatus, each allowing communication via a PSTN coupled by a RF digital radio link.